net: phy: provide phylib stubs for hardware timestamping operations

net/core/dev_ioctl.c (built-in code) will want to call phy_mii_ioctl()
for hardware timestamping purposes. This is not directly possible,
because phy_mii_ioctl() is a symbol provided under CONFIG_PHYLIB.

Do something similar to what was done in DSA in commit 5a17818682cf
("net: dsa: replace NETDEV_PRE_CHANGE_HWTSTAMP notifier with a stub"),
and arrange some indirect calls to phy_mii_ioctl() through a stub
structure containing function pointers, that's provided by phylib as
built-in even when CONFIG_PHYLIB=m, and which phy_init() populates at
runtime (module insertion).

Note: maybe the ownership of the ethtool_phy_ops singleton is backwards,
and the methods exposed by that should be later merged into phylib_stubs.

Signed-off-by: Vladimir Oltean <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

MAINTAINERS

···
 F:	include/linux/of_net.h
 F:	include/linux/phy.h
 F:	include/linux/phy_fixed.h
+F:	include/linux/phylib_stubs.h
 F:	include/linux/platform_data/mdio-bcm-unimac.h
 F:	include/linux/platform_data/mdio-gpio.h
 F:	include/trace/events/mdio.h

drivers/net/phy/Makefile

···
 # dedicated loadable module, so we bundle them all together into libphy.ko
 ifdef CONFIG_PHYLIB
 libphy-y			+= $(mdio-bus-y)
+# the stubs are built-in whenever PHYLIB is built-in or module
+obj-y				+= stubs.o
 else
 obj-$(CONFIG_MDIO_DEVICE)	+= mdio-bus.o
 endif

drivers/net/phy/phy.c

···
 EXPORT_SYMBOL(phy_do_ioctl_running);
 
 /**
+ * __phy_hwtstamp_get - Get hardware timestamping configuration from PHY
+ *
+ * @phydev: the PHY device structure
+ * @config: structure holding the timestamping configuration
+ *
+ * Query the PHY device for its current hardware timestamping configuration.
+ */
+int __phy_hwtstamp_get(struct phy_device *phydev,
+		       struct kernel_hwtstamp_config *config)
+{
+	if (!phydev)
+		return -ENODEV;
+
+	return phy_mii_ioctl(phydev, config->ifr, SIOCGHWTSTAMP);
+}
+
+/**
+ * __phy_hwtstamp_set - Modify PHY hardware timestamping configuration
+ *
+ * @phydev: the PHY device structure
+ * @config: structure holding the timestamping configuration
+ * @extack: netlink extended ack structure, for error reporting
+ */
+int __phy_hwtstamp_set(struct phy_device *phydev,
+		       struct kernel_hwtstamp_config *config,
+		       struct netlink_ext_ack *extack)
+{
+	if (!phydev)
+		return -ENODEV;
+
+	return phy_mii_ioctl(phydev, config->ifr, SIOCSHWTSTAMP);
+}
+
+/**
  * phy_queue_state_machine - Trigger the state machine to run soon
  *
  * @phydev: the phy_device struct

drivers/net/phy/phy_device.c

···
 #include <linux/of.h>
 #include <linux/netdevice.h>
 #include <linux/phy.h>
+#include <linux/phylib_stubs.h>
 #include <linux/phy_led_triggers.h>
 #include <linux/pse-pd/pse.h>
 #include <linux/property.h>
···
 	.start_cable_test_tdr	= phy_start_cable_test_tdr,
 };
 
+static const struct phylib_stubs __phylib_stubs = {
+	.hwtstamp_get = __phy_hwtstamp_get,
+	.hwtstamp_set = __phy_hwtstamp_set,
+};
+
+static void phylib_register_stubs(void)
+{
+	phylib_stubs = &__phylib_stubs;
+}
+
+static void phylib_unregister_stubs(void)
+{
+	phylib_stubs = NULL;
+}
+
 static int __init phy_init(void)
 {
 	int rc;
 
 	rtnl_lock();
 	ethtool_set_ethtool_phy_ops(&phy_ethtool_phy_ops);
+	phylib_register_stubs();
 	rtnl_unlock();
 
 	rc = mdio_bus_init();
···
 	mdio_bus_exit();
 err_ethtool_phy_ops:
 	rtnl_lock();
+	phylib_unregister_stubs();
 	ethtool_set_ethtool_phy_ops(NULL);
 	rtnl_unlock();
 
···
 	phy_driver_unregister(&genphy_driver);
 	mdio_bus_exit();
 	rtnl_lock();
+	phylib_unregister_stubs();
 	ethtool_set_ethtool_phy_ops(NULL);
 	rtnl_unlock();
 }

drivers/net/phy/stubs.c

···
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Stubs for PHY library functionality called by the core network stack.
+ * These are necessary because CONFIG_PHYLIB can be a module, and built-in
+ * code cannot directly call symbols exported by modules.
+ */
+#include <linux/phylib_stubs.h>
+
+const struct phylib_stubs *phylib_stubs;
+EXPORT_SYMBOL_GPL(phylib_stubs);

include/linux/phy.h

···
 #define MII_BUS_ID_SIZE	61
 
 struct device;
+struct kernel_hwtstamp_config;
 struct phylink;
 struct sfp_bus;
 struct sfp_upstream_ops;
···
 			     struct netlink_ext_ack *extack);
 int phy_ethtool_get_plca_status(struct phy_device *phydev,
 				struct phy_plca_status *plca_st);
+
+int __phy_hwtstamp_get(struct phy_device *phydev,
+		       struct kernel_hwtstamp_config *config);
+int __phy_hwtstamp_set(struct phy_device *phydev,
+		       struct kernel_hwtstamp_config *config,
+		       struct netlink_ext_ack *extack);
 
 static inline int phy_package_read(struct phy_device *phydev, u32 regnum)
 {

include/linux/phylib_stubs.h

···
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Stubs for the Network PHY library
+ */
+
+#include <linux/rtnetlink.h>
+
+struct kernel_hwtstamp_config;
+struct netlink_ext_ack;
+struct phy_device;
+
+#if IS_ENABLED(CONFIG_PHYLIB)
+
+extern const struct phylib_stubs *phylib_stubs;
+
+struct phylib_stubs {
+	int (*hwtstamp_get)(struct phy_device *phydev,
+			    struct kernel_hwtstamp_config *config);
+	int (*hwtstamp_set)(struct phy_device *phydev,
+			    struct kernel_hwtstamp_config *config,
+			    struct netlink_ext_ack *extack);
+};
+
+static inline int phy_hwtstamp_get(struct phy_device *phydev,
+				   struct kernel_hwtstamp_config *config)
+{
+	/* phylib_register_stubs() and phylib_unregister_stubs()
+	 * also run under rtnl_lock().
+	 */
+	ASSERT_RTNL();
+
+	if (!phylib_stubs)
+		return -EOPNOTSUPP;
+
+	return phylib_stubs->hwtstamp_get(phydev, config);
+}
+
+static inline int phy_hwtstamp_set(struct phy_device *phydev,
+				   struct kernel_hwtstamp_config *config,
+				   struct netlink_ext_ack *extack)
+{
+	/* phylib_register_stubs() and phylib_unregister_stubs()
+	 * also run under rtnl_lock().
+	 */
+	ASSERT_RTNL();
+
+	if (!phylib_stubs)
+		return -EOPNOTSUPP;
+
+	return phylib_stubs->hwtstamp_set(phydev, config, extack);
+}
+
+#else
+
+static inline int phy_hwtstamp_get(struct phy_device *phydev,
+				   struct kernel_hwtstamp_config *config)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int phy_hwtstamp_set(struct phy_device *phydev,
+				   struct kernel_hwtstamp_config *config,
+				   struct netlink_ext_ack *extack)
+{
+	return -EOPNOTSUPP;
+}
+
+#endif